Electronically stored images are commonly printed from bitmap representations, where a bitmap represents the image in terms of m.times.n array of pixels. Commonly, the pixels in such an array are binary, or representable as 0 or 1, ON or OFF, black or white. A well known binary printing system includes an electrophotographic engine with an exposure device including a laser, which is allowed to expose a photoconductive surface to dissipate a uniformly deposited charge thereon in accordance with the bitmap.
Two types of electrophotographic systems are well known and in common use. In a first system, commonly referred to as a write-white or charged area development system, a laser dissipates charge on the photoconductive surface in areas where development or printing is not desired. Thus, it might be said that white pixels are written to the photoconductive surface. In a second system, commonly referred to as a write-black or discharged area development system, the laser dissipates charge on the photoconductive surface in areas where development or printing is desired. Thus, it may be said that black pixels are written to the photoconductive surface.
While each system has advantages and disadvantages, each is capable of producing a high quality reproduction. However, each has a distinct artifact occurring as a result of the imaging process. Write-white systems tend to shrink or erode black lines, while write-black systems tend to expand black lines. The change in line width can lead to the breaking of letters for small fonts in write-white printers, along with the overall appearance of the printed image appearing to be too light. For write-black printers, the result may be a closing of small letters, along with the overall appearance of a printed image appearing too dark. Knowing these tendencies, some printer manufacturers selling fonts (sets of bitmap images representing alphanumeric character sets) have pre-compensated their fonts to account for one type of printing process or the other. However, in an open systems environment, where write-white and white-black printers are provided for the same users, such pre-compensated fonts are troublesome. Additionally, such pre-compensation does not consider images other than from fonts.
U.S. patent application Ser. No. 07/588,125, entitled "Bitmap Image Resolution Converter Compensating for Write White Xerographic Processes" by L Mailloux, and assigned to the same assignee as the present invention, addresses a particular conversion of a 300 spot per inch (spi) write-black bitmap to a 600 spi write-white bitmap with a process that dilated or expanded the image by a small amount in the context of resolution conversion by bit doubling, with a state selection process that tended to slightly favor a black state over a white state. The described process works, but is not generalized, does not teach conversion from write-white, does not deal with the problem of conversion without an output resolution change, does not teach conversion to either write-black or write-white from a neutral bitmap and does not take into account the need for a variety of compensation amounts. Smoothing and half bit removal are provided at the higher resolution.
U.S. Pat. No. 5,208,871 to Eschbach, and assigned to the same assignee as the present invention, describes a method of converting an image from a first resolution, bit depth and orientation to a second resolution, bit depth and orientation, by emulating the process of printing the bitmap at the first resolution, bit depth and orientation, emulating the process of scanning the bitmap at the second resolution, and orientation, and quantizing from the second resolution to a desired output resolution using error diffusion.